A gangmaster who supplied migrant workers to harvest leeks sold at Sainsbury's has lost his licence after being found guilty of abuse, it emerged yesterday.

An investigation by the Gangmaster Licensing Authority found that about 50 Polish and Slovakian workers at ELS Recruitment Ltd, based in Peterborough, had been threatened, verbally abused, had pay deducted for no apparent reason and had been sacked for questioning their treatment. One worker had allegedly been assaulted by Shamus Paul, the company's director, the GLA said.

Other complaints included forcing workers to use the company's cramped vans to get to work and being made to pay at least £4.50 for the service. Some workers were not given copies of their contracts, while the signatures of others appeared to have been forged by the agency. One worker said the only training they had received was "don't lose your knife".

"ELS ... controlled where some of the workers lived, if they worked, how they got to work and even if they got paid," said Paul Whitehouse, chairman of the GLA. "Anybody who abuses this amount of control over the lives of others deserves to face the consequences."

Paul supplied his workers to Allpress Farms, a Cambridgeshire farm. "My initial reaction was shock," said the farm director, Nick Allpress. "I was very disappointed - this shouldn't be allowed to continue."

The GLA received a call about alleged abuse at ELS on March 6. In the following weeks the agency interviewed witnesses and informed companies further along the supply chain.

Allpress Farms supplied its leeks to Produce World, a major fresh food supplier which in turn sells the vegetables to Sainsbury's. All companies supported the GLA's inspections. Allpress Farms and Produce World have offered continued employment to the workers involved.

Produce World said the company subjected all labour agencies it used to "stringent audits".

"This particular gangmaster had been audited several times, but nothing came to light," said Andrew Burgess, the agricultural director. "We do everything we can to stop this kind of thing from happening. But sometimes someone gets one over you - we're the victim of that."

Judith Batchelor, of Sainsbury's, said in a statement: "Sainsbury's takes the welfare of workers and sourcing with integrity very seriously and this is the type of behaviour that both we and Produce World find totally unacceptable."

Paul and ELS could not be reached for comment last night.

The GLA was set up following the death of 23 Chinese cockle pickers in Morecambe Bay in 2004 to prevent the exploitation of mainly migrant workers who are employed by gangmasters. The agency has so far revoked 53 licences.